The present invention relates to a CVD coated cutting tool insert suitable for machining of metals by turning, milling, drilling or by similar chip forming machining methods. The coated tool insert shows improved toughness behavior when used in interrupted cutting operations.
Modern high productivity chip forming machining of metals requires reliable tool inserts with high wear resistance, good toughness properties and excellent resistance to plastic deformation.
This has been achieved so far by employing a cemented carbide insert coated with a wear resistant coating. The cemented carbide insert is generally in the shape of an indexable insert clamped in a tool holder, but can also be in the form of a solid carbide drill or a milling cutter. Cemented carbide cutting tool inserts coated with various types of hard layers like TiC, TiCxNy, TiN, TiCxNyOz and Al2O3 have been commercially available for many years. Several hard layers in a multilayer structure generally build up such coatings. The sequence and the thickness of the individual layers are carefully chosen to suit different cutting application areas and work-piece materials.
The coatings are most frequently deposited by Chemical Vapor Deposition (CVD) or Physical Vapor Deposition (PVD) techniques. In some rare cases Plasma Assisted Chemical Vapor Deposition (PACVD) has also been practiced.
The CVD-technique is often preferred over PVD as it has several advantages. It allows larger coating batches, produces coatings with good coating thickness distribution on complex shaped inserts, has a high throwing power, can be used to deposit electrical non-conducting layers like Al2O3 and ZrO2. Many different materials can be deposited in the same coating run like, e. g., Al2O3, TiC, TiCxNy, TiN, TiCxNyOz, ZrCxNy and ZrO2.
The CVD technique is conducted at a rather high temperature range, from about 950 to about 1050° C. Due to this high deposition temperature and to a mismatch in thermal coefficient of expansion between the deposited coating materials and the cemented carbide tool insert, CVD produces coatings with cooling cracks and tensile stresses.
PVD processes run at a significantly lower temperature, from about 450 to about 650° C. and are performed under strong ion bombardment which leads to crack free layers with high compressive stresses. The high compressive stresses and the absence of cooling cracks make PVD coated tool inserts much tougher than CVD-coated tool inserts and are therefore often preferred in interrupted cutting operations like in milling.
A noticeable improvement in performance of CVD-coated tool inserts came about when the MTCVD (Moderate Temperature CVD)-technique begun to come into the tool industry about 5-10 years ago. An improvement in the toughness properties of the cutting tool insert was obtained. Today, the majority of tool producers use this technique. Unfortunately, the MTCVD technique is limited only to fabrication of TiCxNy-layers with x being greater than about 0.5, but less than about 0.7 and y being greater than about 0.3 but less than about 0.5 and x+y equal or close to 1. The deposition process here takes place at temperatures in the range from about 700 to about 930° C. It uses a gas mixture of CH3CN, TiCl4 and H2. Today's modern coatings also include at least one layer of Al2O3 in order to achieve high crater wear resistance.
A further improvement in the toughness properties could be obtained when also the deposition temperature for the α-Al2O3 process became possible as disclosed in patent application EP-A-1464727.
Post treatment of coated cutting inserts by brushing or by wet blasting is disclosed in several patents. The purpose is to achieve a smooth cutting edge and/or to expose the Al2O3 along the edge line as, e. g., disclosed in U.S. Pat. No. 5,851,687 and in EP 603 144 or to obtain the Al2O3 as the top layer also on the rake face in those cases when TiN is used as a wear detection layer at the flank face as disclosed in U.S. Pat. No. 5,861,210. Every treatment technique that exposes a surface like, e. g., a coating surface for an impact force such as does, e. g., wet- or dry blasting or ultra sonic shock waves, will have some influence on the stress state (σ) of the coating. However, to considerably relieve tensile stresses in all layers in a CVD-coating structure an intensive surface treatment is required. However, such a treatment may even lead to a too big change in the stress state, e. g., from highly tensile to highly compressive as is disclosed in U.S. Pat. No. 6,884,496, in which a dry blasting technique is used.
For the wet blasting technique, the blasting media, usually Al2O3 grits and water, have to strike the coating surface with a high impulse. The impact force can be controlled by, e. g., the blasting pulp pressure, the distance between blasting nozzle and coating surface, grain size of the blasting media, the concentration of the blasting media and the impact angle of the blasting jet.
Despite these progresses further improvements in toughness properties of CVD-coated tool inserts are very desirable.